1. Field of the Invention
This invention pertains generally to the field of cockpit indicators or display units that provide flight information to the pilot or flight crew of an aircraft, and more particularly to synthetic vision systems.
2. Description of the Related Art
Synthetic Vision Systems (singularly, “SVS”) have been operating for a few years now in a wide range of aircraft; from small, General Aviation airplanes, right up to long range corporate jets; however, these systems are approved or certified for situational awareness only, with the aim of increasing flight safety by providing the flight crew with a real-time, intuitive representation of the external environment. The synthetic scene is rendered from the pilot's eye viewpoint, and typically contains the following: terrain, obstacles, rivers and lakes, and the environment of airports. While certainly enhancing the flight crew's awareness of where they are in space, and where they are relative to terrain, obstacles and the airport, the system cannot currently be used for any operational credit. That is to say there is no difference in operational capabilities between an airplane with an SVS and one without.
With the availability of navigation data provided by satellite navigation systems such as the well-known global positioning system (“GPS”) and publicly-accessible terrain data, synthetic images indicative of the scene outside the aircraft and presented to the pilot on the display unit have proliferated. When waypoint data representative of a flight plan is included in the generating of the synthetic image, it has been possible to display a highway-in-the-sky (“HITS”) upon which the pilot may operate his or her aircraft. The HITS may overlay a background comprised of a synthetic scene depicting sky and ground, where the generation of image data representative of the synthetic scene could have been based upon navigation data such as GPS data. If there are no errors in the navigation data, the synthetic scene will be in conformance with the actual scene outside the aircraft that is viewed by the pilot. If the aircraft is properly aligned with a runway when approaching to land, the HITS will indicate the proper alignment, and the landing area of the runway will be seen. Where the HITS is depicted as a tunnel or tunnel-like, the landing area of the runway will be seen through the tunnel and beyond the tunnel exit. By being able to see the landing area on the other side of the HITS (i.e., the landing area is not blocked from the pilot's view by the HITS or the HITS does not obstruct the pilot's view of the landing area), the pilot is provided with a visual cue that there may be no errors in the navigation data.
If there are errors in the navigation data, the synthetic scene will not be in conformance with the actual scene outside the aircraft that is viewed by the pilot. While a non-conformal synthetic scene may be noticeable to a pilot who, while flying an ILS approach in visual meteorological conditions, has the actual runway landing environment in sight, it may not be noticeable to a pilot who is operating the aircraft in instrument meteorological conditions (“IMC”). When IMC conditions exist, the pilot may not have a visual cue that errors may exist in navigation data. If the generation of the HITS is based upon erroneous navigation data, the pilot may still not have a visual cue that there may be errors in the navigation data because both the synthetic scene and the HITS are based on the same erroneous navigation data. Although the HITS may provide the pilot with an unobstructed view of the landing area and provide the pilot with a visual cue that the aircraft is properly aligned for landing, an aircraft that is actually below the actual glide slope may end up short of the landing area or an aircraft that is actually above the actual glide slope may end up long of the landing area if HITS is relied upon to complete the approach to the runway. Both of these scenarios present unwelcome and potentially hazardous flying conditions because of the erroneous navigation data.